2,2'-di-(1,6,7-trihydroxy-3-methyl-5-isopropyl-8-naphthaldehyde) or so-called gossypol is produced from the bark of cotton roots or from a by-product of processing of cotton seeds. Various pharmaceutical preparations have been developed on the basis of 2,2'-di-(1,6,7-trihydroxy-3-methyl-5-isopropyl-8-naphthaldehyde). In particular, in the form of a 3% liniment it is employed for the treatment of viral diseases of the skin and mucous membranes, as well as keratoconjunctivites.
Known in the art is a method for purification of 2,2'-di-(1,6,7-trihydroxy-3-methyl-5-isopropyl-8-naphthaldehyde) by crystallization thereof from an organic solvent, separation of the formed crystalline precipitate and drying thereof to give the desired product (Journal of the American Chemical Society, vol. 59, No. 5, 1937 /K. N. Campbell, R. C. Morris, R. Adams "The Structure of Gossypol I", p. 1723-1728).
According to this method, 2,2'-di-(1,6,7-trihydroxy-3-methyl-5-isopropyl-8-naphthaldehyde) is dissolved in diethyl ether, an equal volume of petroleum ether added at a temperature of from 30.degree. to 60.degree. C., concentrated under vacuum at 20.degree. C. until the beginning of the formation of a crystalline residue. Then petroleum ether is added again and the mixture is allowed to stand for one hour. The residue is filtered-off and dried to give the desired product. The resulting 2,2'-di-(1,6,7-trihydroxy-3-methyl-5-isopropyl-8-naphthaldehyde) is used in the pharmaceutical industry.
However, the toxicity of 2,2'-di-(1,6,7-trihydroxy-3-methyl-5-isopropyl-8-naphthaldehyde) purified by this method is not always stable. It can surpass the allowable rated values which complicates the use of this compound in compositions of pharmaceutical preparations.
Also known is a method for purification of 2,2'-di-(1,6,7-trihydroxy-3-methyl-5-isopropyl-8-naphthaldehyde) by way of crystallization thereof from an organic solvent, separation of the resulting crystalline residue and drying thereof to give the desired product (SU, A, 1351915).
In this method gossypolacetic acid is dissolved in acetone. The solution is added with activated carbon and stirred for 10-15 minutes. The carbon is filtered-off and washed with acetone on the filter. The filtrate is heated to a temperature of 50.degree.-55.degree. C. and water heated to 55.degree.-60.degree. C. is slowly added thereto under stirring. Two hours thereafter the resulting fine-crystal precipitate is filtered-off, washed with water and dried in vacuum at the temperature of 60.degree. C. for 6 hours to give 2,2'-di-(1,6,7-trihydroxy-3-methyl-5-isopropyl-8-naphthaldehyde) acceptable for use in medical practice.
However, the toxicity of 2,2'-di-(1,6,7-trihydroxy-3-methyl-5-isopropyl-8-naphthaldehyde) purified by this method is also unstable and can surpass the allowable rated values.